only the !Kung and Gwi did not get the majority of their calories from animal sources.

These are both Bushmen tribes. Both have been pushed into marginal areas -- something we always have to bear in mind when talking of modern hunter-gatherers. They eat a lot of plants. Yeah, they would. Where they live there isn't much water. The plants concentrate the water: they eat the plants to get the water. (Sorry, vegetarians: that's just how it is, human needs being what they are.) If they could, they wouldn't live away from water. They don't get to choose.

The upshot is that people don't seem to prefer eating -- what was it Mr. Pollan? -- "mostly plants" if they can choose not to.